Integrated sensors can convert environmental energy into electrical signals, and some of them, as in the case of integrated image sensors, can be used for both sensing and energy harvesting. In the last decade, CMOS image sensors have gained attention due to their inherent advantages of low power and low cost. This is mainly due to the use of standard Complementary Metal Oxide Semiconductor (CMOS) technology which allows for integrating image capture devices as well as complex image processing circuits on a single chip.
CMOS image sensors have a variety of applications in modern portable/mobile electronic systems and sensor networks. These systems are usually powered by batteries or external power supplies. Therefore, power consumption is a major limitation in these portable/mobile systems since the capacity of the batteries often limits their operational time. In the case of sensor network, where the scarcest resource is energy, devices are expected to have a long operational time without human intervention for energy replenishment. Human intervention is undesirable due to the cost of checking a large number of devices. Low power has been typically achieved by using more advanced CMOS technologies featuring low power supply voltage. Low supply voltage, however, is not preferable in CMOS image sensors as it has an enormous impact on imaging performance due to limited signal swing and reduced signal-to-noise ratio (SNR).
Energy harvesting technique can be utilized to exploit energy on-board, thus alleviating the requirement on external battery capacity. For example in CMOS image sensor, a Self-Powered Pixel (SPS) approach that exploits the energy generation capability of integrated photodiodes as shown in FIG. 1, has been previously studied. A photodiode Pd1 (102) is connected between a conventional power supply VDD 103 and a power bus 104 shared by all the pixels in the image sensor. When exposed to incident illumination 106, photodiode 102 converts photons into electron/hole pairs, forming photocurrents that provide extra power to power bus 104. Another photodiode Pd2 (108) and transistors MN1, MN2, and MP1 form a conventional active pixel sensor (APS) structure, in which photodiode 108 operates as the photodetector. MN3 provides a biasing current for signal readout. With the energy generated by the additional photodiode 102, the energy drained from the power supply can be reduced.
However, the existing approach suffers several drawbacks: 1) Significant silicon area is dedicated to the photodetector used for power generation. 2) Before each frame capture, the power photodetector is first charged-up. Poor illumination will elongate this period, thus leading to a very slow operation of the sensor. 3) The SPS cannot operate when the power bus drops below the minimum supply voltage, upon which the bus recharging cycle is invoked.